This invention relates to a system and method for controlling the spindle of an electric motor, and more particularly to a system and method for controlling the spindle of a motor that rotates the platter of a disk drive.
Controlling the speed at which the platter of a disk drive rotates is very important, particularly as storage densities increase and platter size decreases. Thus, in a microdrive—i.e., a drive having a platter diameter of about 1 inch or less—even a small error in angular position resulting from an error in speed control may result in an incorrect sector being read or written. It is therefore a nominal goal to determine disk speed to within 0.01%.
Position, and therefore speed, of a disk drive platter is commonly determined by detecting the back electromagnetic field (back-EMF) generated when one of the rotor poles passes one of the stator poles. For example, it is typical for a disk drive motor to have six poles, so that each pole-pair interaction theoretically signifies 60° of motor rotation. However, in practice, it is difficult during manufacturing to accurately position the poles. Therefore, in practice, some sets of adjacent poles may be closer together than 60°, and other sets of adjacent poles may be further apart than 60°. These offsets may be slight, but may be enough to prevent achieving the desired 0.01% accuracy.
Copending, commonly-assigned U.S. patent application Ser. No. 11/104,683, filed Apr. 12, 2005, which is hereby incorporated by reference herein in its entirety, describes a method and apparatus for deriving calibration data for a motor, and a method and apparatus for controlling a motor using that calibration data. In accordance with those methods and apparatus, one phase of the motor power supply is suppressed (i.e., tristated) during a time duration when back-EMF is expected to be detected, and at the same time one of the other phases is grounded and the third phase is pulled high. If the back-EMF is detected outside that duration, the duration is expanded. This is iterated until the back-EMF falls within the expanded duration.
It has been found that when the one phase of the motor power supply is tristated during back-EMF detection, corresponding current spikes occur in the phases that have not been tristated. Thus, there may be a positive current spike in the phase that has been pulled high, and a negative current spike in the phase that has been grounded. These spikes cause spindle speed jitter and acoustic noise, and moreover increase the peak supply current.
It therefore would be desirable to be able to minimize current variations in the phases of a motor power supply during back-EMF detection.